After setting up a PCR reaction Phil noticed that he'd forgotten to put primers into the mix. When he runs a gel of the PCR reaction after the reaction is finished he sees a positive result. Is this result expected

This is an unexpected result. Absence of primers means that DNA polymerase cannot bind with the template DNA and allow its amplification. If Phil sees a positive band, it should be because of contamination of primers (same set of primer or other) with DNTPs, water, MgCl2, or PCR buffer. This is the reason that a negative run for PCR is always recommended.

they are known to reach a speed of 21 miles per hour

thinking it would be physical characteristics